[Exim] How to set helo_verify for particular helo value?

Hellou all!
In order to struggle with spam i want to configure exim to reject smtp
sessions from hosts (or recepients from hosts) which use
HELO=compuserve.com.
In fact, i need to set helo_verify, but only for one value of helo -
compuserve.com (for example). I dont want to check helo value in all other
cases and i want to accept sessions with no helo specified.

May be there is a possibiliti to write my own check for value of
$sender_helo_name?

Could you give me any advice, how to make it?

    Anton